Tanning is a chemical process, via tannin, for converting skins to leather by making them more durable and more flexible. In leather technology, the functions of the finishing step encompass color leveling, brightness control, as well as providing a protective surface with good resistance to water, chemical attack and abrasion. In this step, the leather will acquire specific properties, particularly in terms of its texture and its appearance. These properties will allow standardization of the leathers obtained from production. Certain compounds act as leveling agents, penetrating agents and dye solubilization aids.
As to the finishing process, typically a layer called a base coating on the leather is first applied. The adhesion of the first layer to the substrate (leather) is very important to the finishing.
Then a second coating is applied, which is harder than the base coating and contains polymer dispersions, for example polyacrylates, and pigments, optionally water-based. This layer is very important with respect to color and thickness of the finishing. Pigments for the coloring and the finishing of leather are, in most cases, inorganic, but organic pigments can also be used. It is intended that this formulation forms a stronger and resistant film when compared to the base coat. After applying the second coating, the leather is usually ironed.
The third coating layer may be a solution of nitrocellulose-based solvent, an emulsion of nitrocellulose or a water-based polyurethane, and generally does not contain pigments. Its content of solids is approximately 10%. This upper layer determines the final properties of leather: it must be as hard and compact as possible to create the desired resistance to friction and so soft as to bend and move quickly without breaking. The third coating layer is probably the most important because it is responsible for the appearance and texture of the leather, in addition to providing resistance against mechanical influences, water and dust. The underlying layers act as a bridge between the structure and the leather surface. The polymers most used in the finishing process are polyacrylates, polybutadiene, and polyurethane. They are generally applied by spray or by a casting process.
There are many known organic solvents that are used in the finishing of leather. Table I below shows the function of some of these solvents.
FamilyProductsKey ApplicationsAlcoholsIsopentanolCo-solventIsobutylic alcoholformulations forSec-butanolleather finishingAcetates ofIsopentyl acetateActive solvents ofalcohols andEthylglycol acetatemedium and lowglycolButylglycol acetateevaporation forethersfinishing leatherdyes and solesKetonesMethyl ethyl ketoneLight active solventfor formulations ofleather, paints andsolesGlycolEthylglycolPenetrating agent forethersButylglycolleather finishing
Among the solvents mentioned above, three have a slower rate of evaporation: ethylglycol acetate, butylglycol acetate and butylglycol, and therefore, they are the last solvent evaporated in the formulations. These solvents evaporate slowly to ensure that the finishing is well leveled and that the film is formed before the system is completely dried.
As mentioned in Table 1, the butylglycol is used as a penetrating agent, which aims to accelerate the penetration of ink on the substrate. Thus this solvent gives better penetration of ink and consequent improvement of adhesion to the substrate.
The known solvents used in the finishing of leather should, apart from solubilizing the resins, also dilute pigments and dyes. There are several types of finishing that are classified according to the dye used, for example, aniline. This maintains the transparency and all visual properties of leather, such as pores, staining, scratch healing and parasite marks. The semi-aniline finishing uses resins, lacquers and auxiliaries for the leather surface resistance. However, in this case a combination of pigments and metal-complex is prepared to disguise defects such as marks from parasites.
The metal-complex dyes are usually solubilized by solvents such as ethylglycol, butylglycol and ethanol. Currently, good solvency power solvents for aniline-metal complexes have been identified.
In the market for leather finishing, there are three disadvantages of using solvent-based formulations: the strong odor, flammability, and health risks that these products may cause. Thus, there is a recognized need for new solvents and formulations that are less aggressive on the health and also less dangerous with respect to explosions.
From now on there are also rules on limiting emissions of volatile organic compounds due to the use of organic solvents in certain activities and installations. These rules have a direct impact on the use of volatile organic compounds in tannery finishing processes, where the consumption of organic solvent is substantial.